The present invention relates to digital video signal processing, and more particularly to pattern generation using a wipe solid generator to provide scaling of a switcher lever arm during wipes between video images that always results in the pattern transition ending or beginning just as the lever arm reaches its limit.
Video switchers or mixers typically include pattern generators as a standard resource. These generators allow wipes from one video signal to another via a changing geometric shape, allow present pattern effects such as spotlights, darkened anonymous faces and split screens, and allow masking effects so that a key is active only within a certain region of a display screen. These pattern generators typically provide a variety of shapes, such as circle, square, diamond, heart, etc., and also include pattern modification controls for position, aspect, rotation, modulation, replication, edge softness and border width.
The wipe patterns are traditionally generated using a wipe solid approach as illustrated in U.S. Pat. No. 4,805,022 issued Feb. 14, 1989 to John Abt entitled "Digital Wipe Generator." To visualize this method, picture an inverted cone, pointing down. By applying a clip and gain function to this three-dimensional shape, a circle is generated. As the clip point moves from the bottom (point of the cone) to the top (base of the cone) of the inverted cone, the circle grows. This is how a circle wipe transition is formed. Many modifiers affect the wipe solid itself. By changing the location of the cone's point on the video screen, the wipe position is modified. By changing the symmetry of the cone, the aspect is changed. Rotation is accomplished by mixing the x and y coordinates of the cone via sine and cosine functions. Modulation is accomplished by varying the position of the cone with various waveforms. Multiple circles are generated by creating a wipe solid consisting of numerous cones. Other modifiers affect the clip and gains. A border is generated by using two clip and gain functions, each with a different clip. The clip separation defines the border width. A low gain produces a soft pattern. By inverting the gain and changing the direction of the clip, the pattern is reversed.
Several problems have traditionally plagued a wipe system. These problems include lever-arm scaling, constant position and consistent sizing.
1. Lever-arm scaling. Each modifier may potentially change the clip values at which the pattern first appears on the screen and vanishes from the screen. This causes difficulty in setting the clip range. Ideally the switcher's lever-arm which controls the clip values causes the pattern to start immediately after it is moved from its end stop, and the pattern vanishes just as the lever-arm travel is complete. Many current switchers lack dynamic lever-arm scaling. Wipes start and end partway through the lever-arm travel, and at extreme pattern positions the wipes "snap" to completion at the end of lever-arm travel.
2. Constant position. One solution to the lever-arm scaling problem is to center the pattern position as the wipe grows. This artifact is often undesirable. It also precludes numerous clip and gain resources from using the same wipe solid since the position of pattern 2 may be modified by the size, i.e., lever-arm position, of pattern 1.
3. Consistent sizing. Some generators provide lever-arm scaling to the detriment of pattern size consistency. A circle in the center of the screen must traverse one-half screen for the pattern to finish the transition. A circle starting from the screen's corner must traverse the entire screen to finish the transition. Therefore, a pattern at fifty percent (50%) lever-arm travel is twice as big if it originates from a corner than if it originates from the screen center. This operation is correct for static pattern position, but the pattern "breathes", i.e., grows or shrinks, as the pattern position is changed to traverse the screen while the lever-arm is not at an end stop. Further a normal wipe features a growing pattern which reveals the new video, while a reversed wipe features a shrinking pattern which conceals the old video. At ten percent (10%) into a circle wipe a small circle is exposed with the new video in the center. For a reversed circle wipe a ten percent transition reveals a large circle with the old video in the center. Pressing the reverse button in the middle of a transition causes the pattern to jump from large to small or vice-versa. What is desired is consistent size regardless of whether the new and old videos are swapped or the direction of movement changes.